It is rapidly becoming a necessity to develop a means to avoid the multipath reflection of TV signals that produces ghosting of images. Ferrite tiles are presently used to control reflections of very high frequency (VHF) TV signals. However, future direct-broadcast satellite TV systems will use frequencies in the range of 10 to 30 GHz. Hence, in anticipation of the use of such systems, there is a need to develop means to attenuate reflections in both the VHF and microwave bands.
The concern to be addressed is how to provide absorption of electromagnetic (EM) radiation over an extremely wide bandwidth and range of incident angles. In the case of normal incidence, there are a variety of single- and multilayer techniques for making good radiofrequency (RF) absorbers (Knott et al. (1985) Radar Cross Section: Its Prediction, Measurement, and Reduction, Artech House, Inc.; Naito and Mizumoto (1987) Electronics and Communications in Japan Part 2, 70(2):12-17; Hashimoto et al (1984) Advances in Ceramics 16:477-683), but all such techniques are effective only over limited bandwidths. Absorption over a broader bandwidth is obtained if the material is graded (i.e., if the material's absorption and wave-impedance properties change gradually away from free-space values as the wave penetrates the material). This approach is particularly attractive at microwave frequencies because the material's required thickness is not too great. On the other hand, synthesizing materials having exactly the right graded electromagnetic properties and producing them economically and in large quantities are generally difficult. Hence, this problem presents difficulties in both absorber design and material synthesis.
EM wave absorbers may be loosely classified as "resonant" or "broadband." Resonant absorbers derive their name from the fact that the conditions for reduction of reflected EM radiation are satisfied, in general, only at one or more discrete frequencies. On the other hand, broadband EM wave absorbers, in principal, provide absorption at all frequencies, but generally become ineffective outside a frequency band as a result of change in material properties with changing frequency. Practically, extant EM wave absorbers designed to cover a wide range of frequencies are generally made up of combinations of EM wave absorbing elements.
The design of broadband absorbers is essentially the design of a lossy matching network between free space and a conducting surface. Providing loss while minimizing reflection is the key in EM wave absorbing material application. Thus, a broadband EM wave absorber design must address two issues: how to promote propagation of incident waves into the material, rather than simply reflecting from the surface, and how to provide the required level of energy absorption once the wave is interior to the absorber.
Two of the oldest and simplest types of absorbers are represented by Salisbury screens and Dallenbach layers. The Salisbury screen (see, U.S. Pat. No. 2,599,944 to Salisbury) is a resonant absorber created by placing a resistive sheet on a low dielectric constant spacer in front of a metal plate. The Dallenbach layer consists of a homogeneous lossy layer backed by a metal plate (G. T. Ruck, editor, Radar Cross-Section Handbook, Vol. II, Chapter 8, Plenum Press, New York, 1970).
In an analysis of the Salisbury screen, it is assumed that an infinitesimally thin resistive sheet of conductance G, normalized to free space, is placed a distance d from a metal plate. Typically, a foam or honeycomb spacer might be used, so spacer dielectric constants in the 1.03 to 1.1 range are achieved. For zero reflectivity, a Salisbury screen requires 377 ohms per square resistance sheet set at an odd multiple of an electrical quarter-wavelength in front of a perfectly reflective backing.
For a screen with 0.5 inch spacing, the reflection coefficient reaches its minimum value (&lt;-40 dB) at a frequency of 5.9 GHz (.lambda.=2 inch). The best performance is obtained for a resistivity of 377 ohms per square, but the performance is still -18 dB reflectivity for a resistivity 20 percent lower (300 ohms per square). However, a resistivity of 200 ohms per square yields barely a -10 dB reflectivity level at the design spacing. The fractional bandwidth for the 377 ohms per square screen at -20 dB reflectivity level is about 25 percent. To achieve similar performance at a lower frequency, the spacing must be increased because the wavelength becomes longer.
The Salisbury screen has been used in varying degrees in commercial EM radiation absorbing materials. However, the rapid oscillations for large spacing would render it ineffective over a wide frequency range. For increased mechanical rigidity, plastics, honeycomb or higher density foams may be used as spacers. To maintain electrical spacing, the resistive sheet would be mounted over a dielectric layer trimmed to an electrical quarter-wavelength in thickness.
It is difficult to fabricate a thin single-layer type electromagnetic wave absorber for the SHF (3-30 GHz) band. Thus, much work has been done in extending the bandwidth of absorbers through the use of multiple layers. The motivation behind this approach is to change the effective impedance with distance into the material to minimize reflections. Two important types of multilayer absorbers are Jaumann absorbers and graded dielectric absorbers.
The bandwidth of a single layer Salisbury screen absorber can be improved by adding additional resistive sheets and spacers to form a Jaumann absorber. To provide maximum performance, the resistivity of the sheets should vary from a high value for the front sheet to a low value for the back. Even better performance is available for the Jaumann absorbers with more sheets, as illustrated by a six-layer absorber (U.S. Pat. No. 4,038,660 to Connolly et al.). A 0.14 inch spacing between layers with a spacer .di-elect cons.=1.03 was used. An average radar cross-section reduction of 30 dB was measured for this design over the range of 7 GHz through 15 GHz, with a minimum of 27 dB at 8 GHz.
As with the Jaumann absorber, where sheet resistance values are tapered to reduce reflection, a graded dielectric can be used to help match the impedance between free space and a perfect conductor. The optimum method for design of such an absorber would be to determine analytically the .mu. and .di-elect cons. required as a function of distance into the material to limit reflection over a given frequency range, subject to incidence angle and thickness constraints. This general form of the problem does not currently have a theoretical or experimental solution.
Typically, practical graded dielectric absorbing materials are constructed of discrete layers, with properties changing from layer to layer. There are several commercially available multilayer carbon-loaded soft foams for broadband absorption. These broadband absorbers provide good absorption in the SHF band. Although commercially available carbon-loaded foams provide good absorption in SHF band, they do not possess required mechanical or thermal properties for particular, e.g., architectural, applications.
There are several commercially available ferrite-based absorbers. Although ferrites are known to show losses at higher frequencies (Hashimoto et al. (1984), supra), there are no ferrite materials, or combinations of ferrites, that can provide the desired performance over the wide frequency range of VHF, UHF and SHF.
There is at least one commercially known ferrite-resistive layer containing broadband absorbers, Eccosorb-UPF from Emerson and Cummings, that combines the low-frequency performance of sintered ferrite with higher frequency absorption capabilities of a lossy foam. This product is composed of a fully sintered ferrite underlayer and various carbon-loaded lossy elastomeric front layers. The sintered ferrite absorbs in the VHF and UHF ranges while the lossy plastic layer provides absorption in the SHF range. The fabrication of this product has been discontinued by Emerson and Cummings.
This product clearly demonstrates that the performance of ferrite absorbers (100 MHz to 1 GHz) and lossy dielectric layers on the top of ferrite layer attenuates the reflection of low frequency waves. However, the lossy foam is not practical for certain applications, e.g., architectural, because of its poor mechanical properties.